Oxidation and diffusion

Which correctly expresses Fick's first law? (J: diffusion flux, D: diffusion coefficient, C: concentration, x: position)

  1. AJ = −D × dC/dx
  2. BJ = −D × dx/dC
  3. CJ = D × C
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Answer: A. J = −D × dC/dx

Fick's first law states that the amount of atoms flowing by diffusion (the flux J) is proportional to the concentration gradient and flows from high to low concentration. The equation is J = −D (dC/dx); the minus sign shows flow toward decreasing concentration.

  • A. Correct. It is proportional to the gradient and flows toward lower concentration.
  • B. The gradient is taken the wrong way around.
  • C. The flux is proportional to the concentration gradient, not the concentration itself.
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